350 The American Geologist. December, won 
is probably all secondary in origin, and is in turn altering to 
penninite. It occurs in thin lamellae with perfect basal cleav- 
age. It is sometimes bent, or even broken. Green biotite oc- 
curs rarely. The pleochroism is very strong. 
«g = dark seal brown green 
«in = seal brown green 
«p = pale brownish yellow or clear yellow pale green 
The alteration to penninite is a very gradual degeneration 
usually affecting, either all of one mass, or all of certain 
lamellse, at once. The change is first to green biotite with 
undiminished birefringence. Very gradually the color fades 
and at the same time the birefringence and the pleochroism 
diminish till finally penninite results. Biotite shoAvs pleo- 
chroic halos about zircon, apatite, and rutile needles. 
Pen?ii?iite is the common ferromagnesian element of the 
rock. It was probably derived in part directly from the 
primary pyroxene, whose existence is clearly indicated by the 
magnetite, which, in separating out, has preserved the trace 
of the original prismatic cleavages at about 93° This is well 
shown in plate XIX, figure 21. The chlorite is clearly derived 
also from biotite, and very possibly in part also from an 
amphibole, in turn derived from a pyroxene. The habit of the 
penninite is lamellar with micaceous cleavage. Twinning 
parallel to the base/'(ooi) occurs. The color and pleochroism 
are slightly variable in intensity, always in green shades with : 
Penninite -|- Penninite — 
Jig = very pale yellowish green green 
to colorless 
«m = green green 
«p = green very pale yellowishgreen 
to colorless 
The absorption is: 
Penninite -\- ?ig < rim — fip 
Penninite — r/g = «in > «p 
Thus the maximum absorption is always parallel with the 
cleavage and elongation, since the bisectrix is always perpen- 
dicular to the basal plane. 
The birefringence is extremely weak, and the interference 
colors are "ultra blue" (and pale yellowish) on account of the 
strong dispersion. 
